Two plasmacytomas of BALB/c mice were shown to produce two immunoglobulin classes. Both tumors were shown to express the two classes within single cells. The IgG2b and IgA paraproteins of one tumor, SAMM 368, were isolated and shown to have differing idiotypes. In addition the IgG1b of SAMM 368 did not express the usually allotype determinants of this class. The idiotypic differences between the paraproteins were found to reflect, in part, major amino acid sequence differences in the amino terminal 40 residues of the light chains. A series of NZB plasmacytomas was analyzed for the frequency of immuno-globulin class distribution. As compared to a similar series of BALB/c tumors, IgG paraproteins were much more frequent and IgA's much reduced in frequency. BIBLIOGRAPHIC REFERENCES: Morse, H. C. III, Neiders, M., Lieberman, R., Lawton, A. R., and Asofsky, R.: Murine plasma cells secreting more than one class of immunoglobulin heavy chain II. SAMM 368-A plasmacytoma secreting IgG2b-kappa and IgA-kappa immunoglobulins which do not share idiotypic determinants. J. Immunol. 118: 1682-1689, 1977. Morse, H. C. III, Goode, J. H. Jr., and S. Rudikoff. Murine plasma cells secreting more than one class of immunoglobulin heavy chain. IV. Sequence differences between kappa-chains of SAMM 368 IgG2b and IgA. J. Immunol. 119: 361-363, 1977.